1. Field of the Invention
The invention relates to solvent casting, which consists of dissolving one or more synthetic resins in an organic solvent, casting the solution onto a suitable substrate, removing the solvent whereby a film is formed on the carrier, and stripping the film from the carrier. Normally the film is wound into rolls.
2. Description of the Prior Art
Solution-cast polymeric films have been used for decades. The most familiar of them are the high quality films used in photographic films. Cellulose ester photographic film base is best known for its dimensional stability and high clarity.
There are many other processes for the formation of films. Calendering, extrusion, plastisol cast systems, and organosol cast systems are the most common. However, solvent casting is the only method that can provide a film which has excellent dimensional stability as well as freedom from pinholes, gels and imperfections. Due to the very low heat history which is inherent in the solvent casting processing, it provides an extended service life to the film.
The solution cast process offers several unique features which conventional fusion processes lack. Extrusion and calendering are processes which melt the polymer and shape the plastic prior to freezing. Plastisol and organosol casting processes involve the melting of the polymer in a plasticizer matrix, after which the solvent action of the plasticizer forms a film. In solvent casting, film formation depends upon solubility, not melting. Thus, a wide range of polymeric alloys can be produced by solvent casting. Because the flowability to form a film is provided by the solvent, a pure resin film can be manufactured without adulteration by heat, stabilizers, plasticizers or lubricants. Only additives which are beneficial to the finished product need to be incorporated with the polymer.
There are disadvantages to solvent casting when compared to other film-forming methods, such as extrusion. Solvent casting obviously requires a solvent, which is in most cases quite expensive. This necessitates a complex solvent vapor recovery and rehabilitation system. Moreover, exposure of personnel to certain solvents is undesirable, and this requires a system that is closed to the atmosphere, especially when temperatures above the boiling point of the solvent are used.
Attempts have been made to solvent cast acrylic films without the use of additives or release coatings applied to the casting substrate. However, these attempts have heretofore been unsuccessful because the films produced were not strippable: the films adhered to the substrate or carrier after the removal of the solvent. Accordingly, the prior art regards acrylic films as not suitable for solvent casting. Current acrylic films are produced primarily by extrusion with its inherent disadvantages. The superior properties of the present films cannot be duplicated in films prepared by other methods, such as extrusion.